_PERMIT FILE - M1981302 ‎(24)‎

• Permit No: M1981302
• IBM Index Class Name: Permit File
• Doc Name: WATER RESOURCES
• Media Type: D
• Archive: No

1960 was selected as the basis for determining normal
evaporation because this represents a period of consistent
and complete records and includes both wet and dry intervals.
These data are contained in Figure C-2. Because the rate
of evaporation from small surfaces , such as pans , is more
than for large open surfaces , such as lakes , it is necessary
to convert pan evaporation measurements to lake surface
evaporation by application of a coefficient. A coefficient
of 0. 78, which is generally applicable to a Colorado sunken
pan, has been applied to the data of Figure C-2 to derive
total lake surface evaporation which is contained in Figure C-3.-
Pan evaporation measurements are limited to the months of
• April through October because during the winter season the
loss rate is relatively low and freezing of water in pans
precludes accurate measurement. Information published by
the U.S. Environmental Data Service indicates that for the
Boulder area May-October evaporation represents 72 . 5% of
the annual total. Using this figure the annual evaporation
was computed as shown in Figure C-3. The evaporation for
November through March was then determined by deducting the
April through October amount from the annual total and is also
shown in Figure C-3.
Net water loss for normal dry and wet year conditions was
determined by reducing total lake surface evaporation, as
C-7